1. Field of the Invention
The invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in a magneto-optic recording and playback system.
2. Description of the Related Art
A conventional magneto-optic recording and playback system (such as a magneto-optic disk drive system) basically comprises a magneto-optic disk, a magnetic head unit and an optical read/write head unit.
Digital signals can be erased, recorded and read from a magneto-optic disk. The magneto-optic disk can therefore be used as a peripheral memory device of a computer system. The magneto-optic disk is usually made of a magnetic material (such as TbFe).
Referring to FIG. 1, when erasing and/or recording data onto the magneto-optic disk, a predetermined region on the disk is heated to a temperature above the Curie temperature (usually at 200.degree. C.). A sharp change in the magnetic characteristics at the heated region then occurs. The heated region is now in an erasable state. The binary data to be recorded on the disk is received by the magnetic head unit. The magnetic head unit generates a magnetic field which is perpendicular to the heated region. The direction of the magnetic field generated by the magnetic head unit depends upon the logic state of the binary data which is received by the same. The direction of magnetization at the heated region aligns with the direction of the applied magnetic field at this stage. When the temperature at the heated region drops below the Curie temperature, the heated region remains magnetized in the direction of the magnetic field which was applied thereto. This illustrates how information is recorded on a magneto-optic disk.
Referring to FIG. 2, when reading information which is stored in a magneto-optic disk, the optical head unit generates a light beam which scans a data track of the disk. The light beam is reflected by the disk and has an angle of polarization which is altered, depending upon the direction of magnetization at the scanned data track. This change in the polarization is known as the Kerr angle.
If the scanned data track was upwardly magnetized (indicating that a binary "1" signal was recorded thereon), the Kerr angle is equal to +.theta., and if the scanned data track was downwardly magnetized (indicating that a binary "0" signal was recorded thereon), the Kerr angle is equal to -.theta.. The logic state of the recorded information can therefore be determined by simply measuring the Kerr angle or the change in the polarization of the reflected beam.
FIG. 3 is an illustration of a conventional magneto-optic recording and playback system. The conventional magneto-optic recording and playback system employs an optical read/write head unit, the elements of which are enclosed by phantom lines in FIG. 3. The operation of the conventional magneto-optic recording and playback system is described briefly as follows:
When recording or erasing information, a laser diode power supply (PS) (1) is first activated so as to operate a laser diode (2). The light beam generated by the laser diode (2) is characterized by a linear (or vertical) polarization, an appropriate wave length (such as 0.78 .mu.m) and a relatively high beam power. The light beam from the laser diode (2) passes through a collimating lens (3), a first beam splitter (4), a second beam splitter (5) and an objective lens (6) and converges to form a light spot having a diameter of approximately 1 .mu.m. The light spot scans a data track of a disk (8) which is rotated by a motor unit (M) (7). The beam power at the scanned data track is at a minimum of 10 mW, thus heating the same so as to exceed the Curie temperature.
When recording binary data ("0" and "1" logic signals) on the disk (8), a signal generator (FTG) (9) is operated simultaneous with the operation of the optical head unit. The signal generator (9) provides current to a magnetic head unit (10) which is structured as an electromagnet. The magnetic head unit (10) is positioned perpendicular to and near the scanned data track of the disk (8). If a binary "1" (or "0") signal was generated during a definite time (t1), current in the positive (or negative) direction flows through the magnetic head unit (10), thereby allowing the magnetic head unit (10) to apply a vertical magnetic field on the scanned data track (A) of the disk (8). The direction of the vertical magnetic field depends upon the direction of current flow through the magnetic head unit (1). Note that when the scanned data track (A) is heated above the Curie temperature, a sharp change in the magnetic characteristics thereat occurs such that the direction of magnetization thereat can be aligned with the magnetic field which is applied by the magnetic head unit (10).
At a definite time (t) which is later than the definite time (t1), the light spot ceases to scan the data track (A) of the disk (8). The temperature at the portion (A) drops below the Curie temperature, and the portion (A) remains magnetized in the direction of the magnetic field applied thereto. This illustrates how recording of binary signals in a magneto-optic disk is conventionally achieved.
When it is desired to erase information contained in the disk (8), the optical head unit is operated in a manner similar to the above described recording operation. That is, the light spot which is generated by the optical head unit is used to scan the data tracks of the disk (8) which are to be erased so as to raise the temperature thereat past the Curie temperature. The signal generator (9), however, provides a direct current signal to the magnetic head unit (10), thereby erasing the contents of the disk (8).
To read the information recorded on the disk (8), a high frequency modulator (HFM) (Il) is used to modulate the power supply (1). This permits intermittent operation of the laser diode (2). Note that the operating frequency of the laser diode (2) is preferably between 100 MHz to 1 GHz. High frequency modulation is employed so as to prevent the light beam which is reflected by the disk (8) from returning to the laser diode (2) and from mixing with he light beam which is generated by the laser diode (2). The reflected light beam, if received by the laser diode (2), can cause instability in the beam power output and in the wave length of the light beam generated by the laser diode (2), thereby affecting the accuracy of the read operation.
The light beam from the laser diode (2) is linearly (or vertically) polarized and passes through the collimating lens (3). The collimating lens (3) arranges the light beam into parallel light rays which occupy a beam area (Z). The light beam passing through the collimating lens (3) has a light intensity (I0). The light beam from the collimating lens (3) passes through the first beam splitter (4). The light beam at a point (B) after the first beam splitter (4) has a light intensity (I1). The light beam from the first beam splitter (4) passes through the second beam splitter (5). The light beam at a point (C) after the second beam splitter (5) has a light intensity (I2). The light beam from the second beam splitter (5) passes through the objective lens (6) and converges to form a light spot. The light spot has a diameter (a) which is approximately 1 .mu.m. The light beam at the illuminated point (A) of the disk (8) has a light intensity (I3). The beam power at the data track (A) of the disk (8) is given by the formula (I3).times.[(.pi.a.sup.2)/4] and is approximately equal to 1 mW.
The light beam which strikes the surface of the disk (8) is reflected downward. Depending upon the magnetization at the scanned data track (A) of the disk (8), the polarization of the reflected light beam is altered by +.theta. or -.theta.. The angle (.theta.) is usually equal to 0.5.degree.. The reflected light beam has a light intensity (I4) and passes through the objective lens (6). The light intensity of the reflected light beam at a point (C) after the objective lens (6) has a light intensity (I5). The light beam from the objective lens (6) reaches the second beam splitter (5) and is split into a first beam which passes through the second beam splitter (5) and a second beam which is reflected.
The first beam from the second beam splitter (5) reaches the first beam splitter (4) and is split into a third beam which passes through the first beam splitter (4) and a fourth beam which is reflected. The third beam passes through the collimating lens (3) and returns to the laser diode (2).
The fourth beam is received by a servo unit (SV) (12). The servo unit (12) derives focusing and tracking error signals from the fourth beam so as to permit the former to control the focusing and tracking of the objective lens (6).
The second beam from the second beam splitter (5) has a light intensity (I6). Note that the polarization of the light beam which strikes the disk (8) is shifted by an angle (.theta.) from a vertically polarized state when the light beam is reflected by the disk (8). The second beam from the second beam splitter (5) passes through a half-wave plate (13). The light beam at a point (D), which is after the half-wave plate (13), has a light intensity (I7), while the amplitude of the light beam thereat is (A7). The half-wave plate (13) causes a 45.degree. shift in the polarization of the light beam passing therethrough. The vertical component of the light beam passing through the half-wave plate (13) has a light intensity (I7) and an amplitude (A8). The horizontal component of the light beam passing through the half-wave plate (13) has a light intensity (I9) and an amplitude (A9). The light beam from the half-wave plate (13) passes through a polarizing beam splitter (15) and is split into a vertically polarized beam and a horizontally polarized beam. The horizontally polarized beam is received by a first photodetector (16), while the vertically polarized beam is received by a second photodetector (17). The light intensity (I9) of the horizontally polarized beam is converted into a first electrical signal (S1) by the first photodetector (16). The light intensity (I8) of the vertically polarized beam is converted into a second electrical signal (S2) by the second photodetector (17). The first and second electrical signals (S1, S2) are received by a differential amplifier (DFA) (18). The (S1-S2) signal output of the differential amplifier (18) represents the information recorded on the disk (8). The following illustrates the relationship between the (S1-S2) signal output of the differential amplifier (18), the light intensity (I0 ) and the Kerr angle (.theta.):
(1) Since only half of the light beam reaching the first beam splitter (4) can pass therethrough, the light intensity (I1)=0.5 (I0).
(2) Since only half of the light beam reaching the second beam splitter (5) can pass therethrough, the light intensity (I2)=0.5 (I1).
(3) Since all of the light beam reaching the objective lens (6) can pass therethrough, the light intensity (I3)=(I2).
(4) Since all of the light beam reaching the surface of the disk (8) is reflected, the light intensity (I4)=(I3).
(5) Since all of the reflected light beam passes through the objective lens (6), the light intensity (I5)=(I4).
(6) Since only half of the light beam from the objective lens (6) passes through the second beam splitter (5), the light intensity (I6)=0.5 (I5).
(7) Since all of the light beam reaching the half-wave plate (13) can pass therethrough, the light intensity (I7)=(I6).
(8) The amplitude (A8)=(A7).multidot.cos (45.degree.+.theta.), while the light intensity (I8)=(I7).multidot.cos.sup.2 (45.degree.+.theta.).
(9) The amplitude (A9)=(A7) sin (45.degree.+.theta.), while the light intensity (I9)=(I7).multidot.sin.sup.2 (45.degree.+.theta.).
(10) The electrical signal (S1) is proportional to the beam power received by the first photodetector (16). The beam power is equal to the light intensity multiplied by the beam area. The electrical signal (S1) is therefore equal to (I8).multidot.(Z).
(11) The electrical signal (S2) from the second photodetector (17)=(I9).multidot.(Z). ##EQU1##
The (S1-S2) signal output of the differential amplifier (18) is inverted by a succeeding electrical stage so as to obtain a voltage signal (S).apprxeq.1/4(I0).multidot..theta..multidot.(Z).
It has thus been shown that the voltage signal (S) is a function of the Kerr angle (.theta.). The information recorded on the disk (.theta.) can thus be read by the magneto-optic recording and playback system since the Kerr angle (.theta.) or the angle of change in the polarization of the reflected light beam depends upon the direction of the magnetization of the scanned data track of the disk (8), which direction of magnetization in turn depends upon the logic state of the recorded binary signal.
The drawbacks of the above disclosed conventional magneto-optic recording and playback system are as follows:
1. The conventional recording and playback system utilizes a high frequency modulator to minimize the adverse effects of the reflected light beam. However, the high frequency modulator can easily affect the operation of nearby electronic devices. Therefore, electromagnetic shielding should be provided so as to suppress electromagnetic radiation of the high frequency modulator. Incorporation of the electromagnetic shielding increases the size, complexity and cost of the recording and playback system.
2. The signal output of the differential amplifier is relatively weak. Note that the magnitude of the signal output of the differential amplifier is approximately equal to 1/4(I0).multidot..theta..multidot.(Z). The weak signal output of the differential amplifier is due in part to the configuration of the optical head unit and can cause improper operation of the succeeding electrical stages.
There is thus a need for a substantially "noise-free" magneto-optic recording and playback system which is smaller in size and which has a stronger electrical signal output.